According to research published in The Lancet, the number of women delivering babies via C-section doctors rose to 29.7 million worldwide in 2015. That is the most recent year for which complete data is available. That’s up from 16 million in 2000, or 12 percent of all births, and significantly higher than the 10-15% considered medically necessary.

The results also highlight a huge gap in childbirth care between rich and poor mothers, with the life-saving surgery unavailable to many women in poor countries and over-used in richer ones.

The procedure can save lives when complications occur, such as bleeding, fetal distress, hypertensive disease, and babies in abnormal position. However, the surgery can also create complications and side effects, including higher risks for future births.

“We call on healthcare professionals, hospitals, funders, women and families to only intervene in this way when it is medically required,” said Marleen Temmerman, an expert from Aga Khan University in Kenya and Ghent University in Belgium who co-led the research.

“In cases where complications do occur, C-sections save lives, and we must increase accessibility in poorer regions, making C-sections universally available, but we should not overuse them.”

The study found C-sections were overused in North America, Western Europe, Latin America, South Asia and the Caribbean and underused in parts the west and central Africa region.